Q. We are about to start our mix design approval for next season. We have been in a market where cement availability has been tight, so we are considering including more ingredients in our designs. How do you suggest we begin our material selection process for our concrete mix designs?

A. Today's concrete mixes are more complex than ever before, according to Peter Taylor, principal engineer and manager of the Materials Consulting practice for CTLGroup, Skokie, Ill. Producers are combining a wider range of cements, supplementary cementitious materials, and chemical admixtures to produce economical concretes that can meet exact performance requirements.

Unfortunately, constituent materials sometimes interact in unexpected ways that adversely affect setting time, workability, and strength development. Among the more common problems that result are premature stiffening, excessive cracking, and poor air-void systems.

Taylor recently completed a Federal Highway Administration study of material incompatibility issues and developed a testing protocol to prevent problems before construction.

“Incompatibility” of concrete materials was defined as interactions between otherwise acceptable materials that result in unexpected or unacceptable performance. The most common problems involve premature stiffening (rapid slump loss) and erratic setting of concrete mixtures (flash set, false set, or delayed setting and strength gain) along with an increased risk of cracking and unacceptable air void systems. Proper consolidation, finishing, texturing, and curing also can be disrupted.

Uncontrolled stiffening and setting can cause serious problems with concrete pavement construction and with other types of flatwork and structures (bridge decks, for example) where the timing of finishing and texturing is critical. These problems may not be noticeable in formed concrete structural elements as long as the concrete is workable enough to be consolidated in place. However, for pavements and structures, rapid stiffening may lead to honeycombing along with incomplete consolidation.